1. Field of the Invention
The present invention generally relates to a channel estimation technique, and more particularly, to channel estimation technique in a frequency domain.
2. Description of Related Art
In a communication system, a channel of poor quality often affects signals transmitted in the channel and thus lowering the quality of the received signals and causing data transmission error. In a so-called frequency-selective fading channel, different frequency bands are affected by different channel effects. In order to compensate the damage of the received signals caused by channel variation, the receiving end must be capable of accurately estimating the channel, so as to achieving reliable coherent detection, and correctly identifying symbols from the received signals.
According to some communication systems, before transmitting data, the transmitting ends thereof are adapted to transmit a preamble signal for the receiving end to perform channel estimation, or timing synchronization, or the like. For example, in a wireless local area network (WLAN) or a digital audio broadcasting (DAB) system, a transmitting end thereof pre-transmits a preamble signal or a reference symbol to a receiving end thereof allowing the receiving end to perform a channel estimation so as to correctly identify a symbol from the received signals.
Because of the importance of channel estimation, methods for the same are being proposed in international periods, conferences, and U.S. patents. For example, S. K. Wilson et al., in “16 QAM Modulation with Orthogonal Frequency Division Multiplexing in a Rayleigh-Fading Environment”, Proc. Of VTC'94, pp 1660-1664; P. Hoeher, in “TCM on Frequency-selective Land-Mobile Fading Channels”, Proc. Of 5th Tirrenia International Workshop Digital Communication, Tirrenia, Italy, September 1991; and U.S. Pat. No. 6,608,863, respectively proposes an orthogonal frequency division multiplexing system. According to such an orthogonal frequency division multiplexing (OFDM) system, after the timing synchronization is completed, a symbol is then transformed by a fast Fourier transform (FFT), and then channel estimation is performed according to characteristic values of each subcarrier of the symbol. Further, U.S. Pat. No. 7,260,055, proposes to perform a channel estimation in a frequency domain with an average of two equivalent long training symbols. However, the methods for channel estimation as mentioned above all rely on one or two symbols of the preamble signal, which cause the channel estimation inaccurate so that the receiving end can not achieve a reliable coherent detection.